1. Field of the Invention
The present invention relates to the production of wet process phosphoric acid from calcium phosphate and sulfuric acid, and, more especially, relates to that process in which, by means of an acid attack upon calcium phosphate, calcium sulfate is formed in the form of its hemihydrate, CaSO.sub.4.1/2H.sub.2 O.
2. Description of the Prior Art
It is known to the art that by means of an acid attack process high concentrations of acid, on the order of 40-55% P.sub.2 O.sub.5 by weight, may be directly attained, while at the same time eliminating the distinct operation of concentration by evaporation.
The reaction leading to the formation of the hemihydrate: EQU Ca.sub.3 (PO.sub.4).sub.2 +3H.sub.2 SO.sub.4 +1.5H.sub.2 O.fwdarw.3CaSO.sub.4.1/2H.sub.2 O+2H.sub.3 PO.sub.4
has been studied, especially by Nordengren, e.g., U.S. Pat. No. 1,776,593 (1930), then by Dahlgren, see Chapter 2, Volume I of A. V. Slack, Phosphoric Acid, Dekker, N.Y. (1968). An industrial process is described in U.S. Pat. No. 19,045 by Larsson, or French Pat. No. 709,848; however, the difficulties militating against successful operation are numerous; same are dur particularly to poor yields of the attack reaction in the presence of unreacted P.sub.2 O.sub.5, with the tendency of blocking the reaction through the deposition of calcium sulfate on the granular phosphate; also, the presence of P.sub.2 O.sub.5 rendered insoluble in the form of dicalcium phosphate co-crystallized with calcium sulfate; problems of filtration and washing in the concentrated medium too have been reported, in French Pat. No. 709,848 (1931).
Inherent difficulties in reaction yields also exist when the phosphate is treated under conditions giving rise to gypsum formation and solutions have been determined to exist, particularly in relation to the conditions of the reaction with respect to temperatures and concentrations, which result in satisfactory continuous operations. Equipment has also been proposed providing good conditions for the crystallization of gypsum on an industrial scale.
However, such difficulties exist on an even larger scale in the range theoretically leading to the hemihydrate. Further, in the hemihydrate range, the high concentration of the acid to be separated and, consequently, its high viscosity, together with the slight amount of water admitted to the reaction, represent unfavorable conditions for satisfactory crystallization and good washing. The difficulties of filtering and washing due to mediocre crystallization are such that a number of industrial processes are bottomed on an imperfect separation of the hemidrate, or eliminate washing or even the separation itself; this is the case with those processes illustrated in the U.S. Pat. No. 3,505,013. However, filtration and intensive washing are important. To these problems of filtration due to the poor crystallization of the hemihydrate and the viscosity of the acid, is added the problem of corrosion resulting from the elevated temperatures employed.
On the other hand, the hemihydrate is stable within narrow limits only; consequently, upon a slight drop in temperature, for example, the hemihydrate tends to be converted or transformed into gypsum, resulting in the phenomena of solidification and scaling.
In attempting to eliminate these difficulties and obtain good crystallization of the hemihydrate, most of the methods known at the present time are predicated upon performing the phosphate reaction progressively, in several successive stages, as per Larsson in 1931 in French Pat. No. 709,846; a preliminary attack by dilute phosphoric acid is described in U.S. Pat. Nos. 3,552,918, 3,653,826 and 3,418,077, British Pat. No. 1,250,191; a fractioning of the sulfuric acid supply is described in South African Patent No. 68/0570.
It is generally recognized that satisfactory crystallization may be obtained when hemihydrate crystals are formed initially in a medium having a SO.sub.4.sup.-- content less than stoichiometric, particularly in a medium containing intermediate reaction products, such as monocalcium phosphate; the first reaction medium obtained is then transferred to a subsequent reaction zone, where the SO.sub.4.sup.-- content is raised to stoichiometry or beyond, causing the precipitation of all of the calcium in the form of hemihydrated calcium.
In all of these processes, the growing crystals are thus placed in or exposed to the presence of successively differing conditions of concentration and temperature.